The present invention relates to a reflective liquid crystal display with little flicker and screen burn-in for high picture quality and a method of assembling the reflective liquid crystal display.
Thanks to compactness and low power consumption compared to cathode ray tubes (SRT), liquid crystal display have often been used in office automation (OA) equipment, such as, notebook computers, personal data assists (PDA) and word processors, and also mobile phones.
A known reflective liquid crystal display is disclosed in Japanese Unexamined Patent publication No. 09 (1997)-269482.
The known liquid crystal display is equipped with: a multiple number of TFTs (Thin Film Transistors) arranged in a matrix on a substrate made of a conductive material, via an insulating layer; an interlayer insulating film that covers the TFTs; a reflective pixel electrode connected to the drain of the corresponding TFT via an electrode; a transparent electrode provided over the reflective pixel electrode so that the two electrodes face each other, and a liquid crystal sealed between the two electrodes.
In operation, a read light incident into the transparent electrode is modulated by light modulation in accordance with a video signal provided from the TFT, reflected by the reflective pixel electrode and emitted from the transparent electrode for image display.
Formed on the reflective pixel electrode and the opposing transparent electrode are a pair of alignment films by which the liquid crystal is oriented.
The reflective pixel electrode is mostly made of a material exhibiting high reflectivity, such as, aluminum or silver. In contrast, the opposing transparent electrode is mostly made of a transparent and conductive material, such as, an indium tin oxide (ITO) film. These materials for the two opposing electrodes exhibit different work functions.
The different work functions cause difference in contact potential difference between contact potential appearing across the reflective pixel electrode and its corresponding alignment film and that appearing across the transparent electrode and the other alignment film.
The contact potential difference further causes D.C. components to be applied to the liquid crystal, which results in flicker on displayed images or screen burn-in due to segregation of ionized impurities generated from the liquid crystal when the same image is displayed for a long time, thus giving adverse effects to image quality.
It is a known fact, in reflective liquid crystal displays, that difference in work function between a reflective pixel electrode and an opposing transparent electrode within ±2% causes little flicker and screen burn-in.
Japanese Unexamined Patent publication No. 2002-365664 discloses a reflective liquid crystal display based on this fact.
This known liquid crystal display is equipped with: a multiple number of TFTs formed on a glass substrate; a flattening film that covers the TFTs; a reflective pixel electrode, made of aluminum or silver, formed on the flattening film and connected to the drain of the corresponding TFT via a contact hole; an ITO-made transparent electrode provided over the reflective pixel electrode so that the two electrodes face each other; an ITO-made transparent pixel electrode provided on the reflective pixel electrode so that the transparent pixel electrode faces the transparent electrode; a pair of alignment films formed on the transparent electrode and the transparent pixel electrode; and a liquid crystal sealed between the alignment films.
It is disclosed that polarization on the interface between the transparent pixel electrode and its corresponding alignment film and that on the interface between the transparent electrode and the other alignment film are equal to each other so that no D.C. components are applied to the liquid crystal, which offers high image quality with very little flicker.
The known liquid crystal display disclosed in Japanese Unexamined Patent publication No. 2002-365664, however, has the following problems:
As illustrated in FIG. 1, when light is incident in a transparent electrode 26, a light component reflected by a reflective pixel electrode 27 and that reflected by a transparent pixel electrode 28 formed on the electrode 27 interfere with each other. This is because the material for the reflective pixel electrode 27 is aluminum or silver that exhibits high reflectivity but refraction index different from ITO.
The interference causes decrease in reflected light emitted from the transparent electrode 26. In other words, a bright image is displayed when a positive peak level of the light component reflected by the reflective pixel electrode 27 and that of the light component reflected by the transparent pixel electrode 28 are superposed on one another whereas a dark image is displayed when negative peak levels of these light components are superposed on one another.